Use of BAdI /SAPSLL_CUHD_PROP for text fields

Hello together,

I have to create a data proposal for text fields on custom document header. Therefor I found thread data proposal for a text on the text tab that worked great! Problem here for us is, that the text field in the data proposal can only take 100 characters, that´s too small for our needs.

Therefor I thought to just insert a "code" including a text-id and text-name from SO10. With this I thought in a BAdI implementation of /SAPSLL_CUHD_PROP I can manipulate the data proposal, read with the "code" from data proposal my text out of SO10 and then populate that text in the target field of the custom document header.

Anyway, I get stucked with the step of implementing the BAdI. I cannot find the text fields there in the data structures? Is this correct? How are you using the BAdI /SAPSLL_CUHD_PROP, maybe you have an example?

Any other suggestions how to achieve to get default text data (~500 digits) (based on FTO) into my custom document header is appreciated!